assigning _charset to NULL gives a segfault, this fixes that
assigning _charset to NULL gives a segfault, this fixes that

bladams/pymssql,bladams/pymssql,Aloomaio/pymssql,google-code-export/pymssql,Alwnikrotikz/pymssql,AlanZatarain/pymssql,Aloomaio/pymssql,zerolugithub/pymssql,pymssql/pymssql,JimDennis/pymssql,MunDaesik/pymssql,msabramo/pymssql,ramiro/pymssql,Aloomaio/pymssql,AlanZatarain/pymssql,thegooglecodearchive/pymssql,zerolugithub/pymssql,bladams/pymssql,msabramo/pymssql,zerolugithub/pymssql,pymssql/pymssql,google-code-export/pymssql,djhenderson/pymssql,klothe/pymssql,MunDaesik/pymssql,thegooglecodearchive/pymssql,JimDennis/pymssql,google-code-export/pymssql,djhenderson/pymssql,ramiro/pymssql,Alwnikrotikz/pymssql,MunDaesik/pymssql,klothe/pymssql,Alwnikrotikz/pymssql,ramiro/pymssql,thegooglecodearchive/pymssql,AlanZatarain/pymssql,djhenderson/pymssql,klothe/pymssql,JimDennis/pymssql,msabramo/pymssql